The present invention generally relates to substrate processing method and more particularly to a nitridation method of an oxide film formed on a silicon substrate surface.
With progress in the art of device miniaturization, production of ultra-miniaturization semiconductor devices having the gate length of 0.1 μm or less is becoming possible these days.
In order to improve the device operational speed by way of decrease of the gate length in such ultra-miniaturization semiconductor devices, there is also a need to decrease the thickness of the gate insulation film in accordance with scaling law. Thus, in the case of using a conventional thermal oxide film as the gate insulation film, it is necessary to reduce the thickness of the gate insulation film to a thickness equal to or smaller than the conventional thickness of 1.7 nm. However, when the thickness of the oxide film is decreased like this, there arises a problem in that the gate leakage current through the oxide film is increased as a result of tunneling effect.
Because of this problem, attempts has been made to use a high dielectric film (so-called high-K dielectric film) such as Ta2O5 or HfO2, ZrO2, for the gate insulation film in place of the conventional silicon oxide film. However, the nature of such high dielectric film is very much different from those of the silicon oxide film used conventionally in the semiconductor technology, and there remain numerous problems to be solved before it becomes possible to use these high dielectric films as a gate insulation film.
Contrary to this, a silicon nitride film or a silicon oxynitride film is the material used with the conventional semiconductor process and is thought as being a promising material for the gate insulation film of high-speed semiconductor devices of the next generation, in view of the specific dielectric constant thereof of 1.5–2 times as large as that of a silicon oxide film.
Conventionally, a silicon nitride film has generally been formed by a plasma CVD process on an interlayer insulation film. However, such a CVD nitride film generally contains a large number of defects and is characterized by large leakage current. Thus, a CVD nitride film is not suitable for a gate insulation film. In fact, there has been no attempt made to use a nitride film for a gate electrode.
On the other hand, there has been proposed a technology recently that converts the surface of a silicon oxide film into an oxynitride film by N radicals or NH radicals generated by introducing a gas containing nitrogen such as a nitrogen gas, or nitrogen and hydrogen gases or an NH3 gas, into rare gas plasma such as microwave-excited Ar or Kr plasma; The oxynitride film thus formed provides a small thermal oxide-film equivalent film thickness and a leakage current characteristic that surpasses a thermal oxide film. Thus, it is thought that such a film is a promising material for the gate insulation film of the next generation high-speed semiconductor devices. The oxynitride film thus formed is also chemically stable and can suppress, in the case a high-dielectric film is formed on the oxynitride film, the diffusion of metallic elements in the high dielectric film through the oxynitride film, and thus, the reaction between the high-dielectric film and the silicon substrate caused by such diffusion. Further, there is also proposed a technology of directly nitriding the surface of a silicon substrate by such microwave plasma.
Meanwhile, it has been known conventionally that nitrogen can be introduced into an oxide film by way of heat treatment in nitrogen ambient or by way of ion implantation of nitrogen ions. With such a method, however, it is known that the nitrogen atoms thus introduced tend to concentrate in the vicinity of the interface between the silicon substrate and the oxide film. As a result, in the case such a conventional oxynitride film is used for the gate insulation film of a MOS transistor, there arise problems such as the change of threshold voltage or deterioration of mutual conductance caused by the formation of interface states.
Because of similar reasons, there would be caused a problem also in the case of the oxynitride film formed by the N radicals or the NH radicals in that not only the desired improvement of the semiconductor device characteristics is not attained but also the degradation of the device characteristic is caused, unless the distribution of the nitrogen atoms in the film is controlled appropriately.
Further, in the case an oxynitride film is formed by nitriding an oxide film by using high-energy plasma such as inductively coupled plasma (ICP), no oxynitride film of desired property is obtained unless a plasma damage recovering process is conducted by applying an annealing process after the nitridation processing. However, such a recovery annealing process is not desirable in view of the fact that such an annealing process is an excessive process and that there may be caused further oxidation of the oxynitride film, resulting in increase of thickness of the oxynitride film.